1. Field of the Invention
The present invention relates to variable-frequency inverter microwave ovens and methods for controlling the same, and more particularly to a variable-frequency inverter microwave oven and a method for controlling the same, wherein a constant-level voltage is supplied to a magnetron, which generates electromagnetic waves in the microwave oven, so that a drive duration of the magnetron can be lengthened, thereby improving heating efficiency and reliability of an inverter operation.
2. Description of the Related Art
FIG. 1 is a block diagram showing the construction of a conventional inverter microwave oven. A description will hereinafter be given of the construction and operation of the conventional inverter microwave oven with reference to FIG. 1.
The conventional inverter microwave oven is a cooking apparatus using dielectric heating, which is a kind of high frequency-based device. The dielectric heating is to radiate microwaves to food received in the oven to vibrate molecules of water in the food, thereby heating the food. This dielectric heating heats food beginning with the inside thereof in a different manner from common heating methods which heat food beginning with the surface thereof. To this end, the inverter microwave oven typically utilizes microwaves having a wavelength of about {fraction (1/10)} that of TV broadcast waves.
A magnetron 2, which is a particular two-pole vacuum tube, is used as a heating source for generating microwaves. The magnetron 2 concentrates the microwaves on food through an output antenna to heat the food. An internal space of the oven where the microwaves resonate is called a cavity 1, which includes a turntable for rotating the food to heat it evenly.
The microwave oven further comprises a cooking command input unit (not shown) provided at the outer surface of the oven for enabling a user to input a cooking command, a display unit (not shown) for displaying the remaining time or elapsed time in response to the cooking command inputted through the cooking command input unit, and a door (not shown) for opening/closing the front of the cavity 1.
A commercial alternating current (AC) voltage source 4 is provided to supply a commercial AC voltage of 220V-60 Hz to a general home, in which the microwave oven is installed, to drive the magnetron 2. An inverter 3 is also provided to convert the commercial AC voltage supplied from the commercial AC voltage source 4 into a high-power direct current (DC) voltage of about 4000V or more and supply the converted DC voltage to the magnetron 2. As a result, the magnetron 2 is driven to heat and cook food in the cavity 1.
The construction and operation of the conventional inverter microwave oven will hereinafter be described in detail with reference to FIG. 2, which is a functional block diagram of the conventional inverter microwave oven.
The conventional inverter microwave oven comprises a DC voltage generator 5 for rectifying and smoothing the commercial AC voltage supplied from the commercial AC voltage source 4 and outputting the resulting DC voltage to a switching device 6. To this end, the DC voltage generator 5 is provided with a bridge diode.
The switching device 6 acts to perform a switching operation based on the DC voltage from the DC voltage generator 5 under control of a microcomputer 9 to generate an AC voltage, and output the generated AC voltage to a magnetron drive voltage generator 7. To this end, the switching device 6 includes a plurality of switches turned on/off in response to the DC voltage from the DC voltage generator 5. The magnetron drive voltage generator 7 functions to convert the AC voltage from the switching device 6 into a high-power DC voltage appropriate to the driving of the magnetron 2 and output the converted DC voltage to the magnetron 2.
The conventional inverter microwave oven further comprises a protection circuit 8 for measuring the level of the DC voltage from the DC voltage generator 5, inputted to the switching device 6, and, if the measured DC voltage level is higher than or equal to a predetermined threshold voltage level, turning off the switching device 6 to prevent the switches thereof from being damaged. For the convenience of description, the DC voltage generator 5, switching device 6, magnetron drive voltage generator 7, protection circuit 8 and microcomputer 9 are shown in FIG. 2 to constitute the block of the inverter 3.
However, the commercial AC voltage for the home supplied from the commercial AC voltage source 4 has voltage/current characteristics varying with regions/time zones. A DC voltage with a ripple component, generated by the DC voltage generator 5, varies with a variation in the AC voltage, resulting in a variation in an AC voltage generated by the switching device 6 because it performs a switching operation based on the DC voltage. As a result, a high-power DC voltage, generated by the magnetron driving voltage generator 7 according to the AC voltage from the switching device 6, is subject to a variation.
Accordingly, the output power of the magnetron 2, which is a heating source for cooking food, varies with the variation in the commercial AC voltage. For this reason, even though the user inputs the same cooking command, he/she cannot obtain a desired cooking result because the cooked state of food is not uniform. Consequently, the user has the trouble of having to additionally heat the food.
In other words, the magnetron 2 is driven only when it is applied with a high-power DC voltage of a reference level or more from the magnetron drive voltage generator 7. In this regard, where the magnetron 2 is applied with a DC voltage of less than the reference level due to a variation in the commercial AC voltage, it is not operated as the heating source, leading to a reduction in drive time thereof.
The reduction in the drive time of the magnetron 2 in turn results in a reduction in heating efficiency, thereby causing heating power not to be maintained at a constant level. Besides, in the case where the commercial AC voltage abruptly increases or decreases, the switches of the switching device 6 may be damaged, resulting in a degradation in durability of a product.